Human granulocytic ehrlichiosis (HGE), an emerging human infectious disease, is increasingly being recognized in the United States. Serological and PCR studies suggest that HGE infection also exists in Europe. HGE is caused by infection with an obligatory intracellular bacterium, HGE agent. Comparison of 16S rRNA gene sequences and ultrastructure indicates that the HGE agent is closely related to Ehrlichia phagocytophila, the agent of tick-borne fever, and E. equi, the agent of equine ehrlichiosis. The HGE agent is transmitted by the Ixodes sp. tick, and the white-footed mouse is considered to be the major reservoir of the HGE agent in the United States.
HGE infection is characterized by the presence of ehrlichial inclusions called morulae in human or animal peripheral blood granulocytes. The symptoms of HGE include chills, headache, myalgia, hematological abnormalities, including leukopenia and thrombocytopenia. HGE frequently requires prolonged hospitalization. When treatment is delayed due to misdiagnosis, HGE can be fatal.
Although several laboratories have used IFA testing, acute-phase blood smear, nested PCR, and culture isolation for diagnosis of HGE, each of these diagnostic tests has disadvantages. IFA testing using the HGE agent or E. equi-infected cells has been most widely used for serodiagnosis of ehrlichiosis. IFA requires a tissue culture system for preparation of HGE-infected cell antigen slides, fluorescent microscope, and trained persons especially for evaluation of the serum reactivity to the antigen on the slide. The nested PCR, while better suited to early diagnosis of HGE than IFA testing, requires thermocycler and trained personel, and the reagents are relatively expensive. The sensitivity of culture isolation in HGE diagnosis seems to be relatively lower than that of IFA test and the nested PCR. Moreover, diagnosis by culture isolation requires serologic evidence, PCR, or 16S rRNA gene sequence for confirmation of the identity of new isolates.
Accordingly, a convenient and sensitive method with high specificity for the diagnosis of HGE infection is still desirable.